As computers grow in speed and shrink in size, power consumed within the computer per unit volume (power density) increases dramatically. Thus, it becomes essential to dissipate the heat generated by electronic components within the computer during operation to ensure that the components remain within their normal operating temperature ranges, since otherwise the components will fail immediately or will have a significantly shorter lifetime.
One of the most common techniques of dissipating heat from a component of a computer is to directly apply a relatively high velocity air flow across the surface of the component and heatsinks to force cooling the component. This raises the convective heat transfer coefficient for the surface of that component, thereby increasing the convection cooling. Most computers are provided with fans to promote force cooling, thus increasing the temperature differential between the surface of the component and the surrounding air to increase the efficiency of the heat transfer.
Of all components in a computer, the microprocessor central processing unit (“CPU”) liberates the most heat during operation of the computer. It has therefore become common practice to provide a heatsink for the CPU to increase the heat-dissipating surface area for more effective cooling. In addition to the heat sink associated with the CPU, a dedicated CPU cooling fan is often used to provide force cooling and air exchange to dissipate the heat generated by the CPU.
Electronic racks comprising stacks of electronic system chassis or multi-blade server chassis are becoming increasingly popular. Such architecture allows to package processors along with their associated electronics in removable drawer or blade configuration disposed within a rack. In general, an electronic rack houses a plurality of thin, modular electronic printed circuit boards PCBs, possibly but not exclusively referred to as server blades. Each PCB may include one or more processors, memory, network controllers, and input/output (I/O) ports, and functions as a server, possibly dedicated to a particular application. In a mainframe environment, one does not necessarily speak of stacking of blade server chassis but the architecture is similar from the point of view of stacks of removable PCB drawers.
Blade servers or PCB drawers in some aspects offer many advantages, e.g., they contain hot-pluggable parts. The hot-plugging technique, sometimes known as hot swapping, enables the repair or replacement of computer systems without disturbing the operation of the total system, i.e., the blades or PCB can be removed without system shutdown. However, there are also disadvantages. One such disadvantage consists in the fact that cooling of these systems is very much complicated due to the amount of CPUs per rack. Thus, more and more so called hotspots, i.e., heat emitting devices share the same cooling flow, which, in general, has for a long time reached its cooling capacity.
To satisfy these increased cooling demands, cooling systems will have to be enhanced taking into account the physical coherences. Currently, the components on the blade are cooled down after a strong heating by convection of air. Due to the enormous generation of heat, air outlet temperatures of more than 50° C. with extremely high air speeds and noise emission will result. This can be controlled by placing a water cooler directly within the air flow, which deprives the strongly warmed air of heat again. Another cooling concept is the so called heat pipe technology, where chips, e.g. CPU, are connected to an air cooled heat sink by heat pipes. A typical heat pipe consists of a sealed hollow tube. A thermo conductive metal such as copper or aluminum is used to make the tube. The pipe contains a relatively small quantity of a “working fluid” or coolant with the remainder of the pipe being filled with vapor phase of the working fluid, all other gases being excluded.
On the internal side of the tube's side-walls a wick structure exerts a capillary force on the liquid phase of the working fluid. This is typically a sintered metal powder or a series of grooves parallel to the tube axis, but it may in principle be any material capable of soaking up the coolant. If the heat pipe has a continual slope with the heated end down, no inner lining is needed. The working fluid simply flows back down the pipe. This type of heat pipe is known as a Perkins Tube. The advantage of heat pipes is their great efficiency in transferring heat. They are actually a vastly better heat conductor than an equivalent cross-section of solid copper. But as the heat is finally transported to air, this technology has no principal advantages over direct air cooling.
At least in the professional field of high-performance computers, liquid cooling systems become more and more accepted, their decisive advantage being particularly their high heat intake capacity which is powers of tens higher as compared to air. With respect to blade centers or mainframe computer, two possibilities of liquid cooling can be distinguished. When using rack cooling, the air is cooled by a liquid cooled heat exchanger before the air can leave the rack, or the air is circulated inside the rack in a closed circuit cooled by a liquid cooled heat exchanger. However, this method requires a high energy and special air conditioning systems.
With direct liquid cooling, cooling liquid is directly applied to the respective heat emitting device (electronic components). There is either a single liquid circuit, or a system with primary and secondary liquid circuits. Such a system is disclosed in WO 2006/005325, the invention relating to a heat exchange system for electronic devices, preferably data processing devices, comprising high-performance processors or having high processor density. The heat exchange system comprises essentially a primary cooling circuit and a secondary cooling circuit, both being thermally associated to the one or more processor unit(s). The secondary cooling circuit is configured as a completely closed system, the coolant in the secondary cooling circuit being driven exclusively by mechanical or magnetic coupling with the flow drive of the primary cooling circuit. Such alternative has the advantage to allow hot swapping but requires very technically challenging architecture to combine a secondary cooling circuit on each PCB which makes such solution very cost-intensive. Water is circulating inside the possible blade housing and the system shows great installation height. Furthermore, defective pipes or hoses can cause outflow of water that leads to damages to the server (breakdown of the processor and damage to the electronic device, respectively).
In US 2008/0225478 is disclosed a heat exchange system for blade server systems. The heat exchange system comprises first heat sinks associated to each of said plurality of server blades, whereby the first heat sinks are adapted to collect heat emitted from heat emitting device on said associated server blade via heat pipes. The heat exchange system comprises also a liquid cooled second heat sink associated to said blade center whereby said first heat sinks are connected to the second heat sink by thermal coupling. Such solution is based on the use of heat pipes which are very cost-intensive. Furthermore, the disclosed coupling between the first and the second heat sink seems not to be reliable in a satisfactory way.